The present invention relates to an optical apparatus for measuring the size distribution of sample particles through an analysis of the intensity distribution of light scattered by the sample particles kept suspended in a medium fluid.
The basic constitution of a conventional exampler of such an apparatus consists, as is illustrated in FIG. 5, essentially of a laser system (not shown) for making a light beam L, a transparent sample vessel 1 made, for instance, as a flow cell through which flows a medium with sample particles dispersed therein, a Fourier transformation lens 2, a photosensor assembly 3 made up of concentrically arrayed semi-circular photosensors 33, and a computer 4. With the sample vessel 1 irradiated by the light beam L, the sample particles dispersed in the flowing medium scatter the light beam L in every direction. In this apparatus, part of the light scattered forward is detected by the photosensors 33, which output data signals reflecting the intensity distribution of the forward scattered light. The computer 4 processes the data signals to derive the size distribution of the sample particles according to an algorithm predetermined on the basis of the theory of the Fraunhofer diffraction or of the Mie scattering.
As to this type of apparatus it is well known that the precision of particle size distribution measurement is increased with increase in the angular region of detectable scattered light. Therefore, more precise measurement capable of obtaining information on the particle size of the order of 0.1 .mu.m can be achieved by providing the apparatus additionally with photosensors for detecting sideward (scattering angle: 90.degree.) and backward (scattering angle larger than 90.degree.) scattered components of light. However, there are many problems to be solved in detecting also the light scattered at large angles.
The intensity of scattered light decreases with increase in the angle of scattering. The intensity, though it depends also on that of the incident light beam L, usually decreases to such a very low level as 1 lux or lower for large scattering angles reaching 90.degree. or larger. In addition the precision measurement of particle size distribution necessiates the detection of scattered light with an accuracy of 99% or higher. Therefore, small temperature-dependent zero-level drifts of the photosensors used can not be ignored, paraticularly when the measurement is made, as is often the case, for samples which must be kept at a temperature much higher or lower than an ambient temperature. Typical temperature-dependent characteristics of a photodiode commonly used in the photosensores are shown in FIGS. 3 and 4 standing, respectively, for the dark current and for the short-circuit current. In the case of the previously mentioned conventional apparatus, each time a new measurement operation is commenced with a new sample, the apparatus has its zero-level readjusted corresponding to an ambient temperature at that time, but a possible very small zero-level drift due to an ambient temperature variation which may arise during a relatively short period of time of the measurement has been ignored, because the apparatus detects only small-angled forward scattered light having a relatively strong intensity.